A typical light weight will bicycle wheel comprises a rim, a hub assembly and a plurality of spokes that connect the hub with the rim. This basic design has been in use for well over a century and has proven to be quite successful.
Most bicycle rims are made of a metallic material, such as aluminum, although recently advanced composite materials, which offer very high strength to weight ratios, have begun to become popular on high-end racing bicycles. Advanced composite materials which utilize a combination of high strength reinforcing fibers and a polymeric matrix have strength to weight ratios that far exceed most metallic materials. Accordingly, bicycle rims fabricated from advanced composite materials can be made significantly lighter than comparable metallic rims. Furthermore, the low weights of advanced composite materials, about two thirds the weight of an aluminum alloy, permit the fabrication of rims having a much greater depth to width ratio without incurring a weight penalty. Rims having a large depth to width ratios have been found to be advantageous in reducing aerodynamic drag.
The advanced composite materials most commonly utilized in bicycle rims comprise carbon or graphite fiber reinforced with an epoxy matrix. However, other reinforcing materials may be used, such as but not limited to fiberglass, aramid fiber, and boron fiber. It is further appreciated that the advanced composite material of a rim can comprise more than one type of reinforcing fiber. Further, there are many different types of carbon or graphite fiber having different physical properties that can be utilized in a suitable advanced composite material. Besides epoxy, which is a thermosetting polymer, other suitable thermosetting polymers can be utilized as well as thermoplastic polymers. Like carbon fiber, there are also a wide variety of different epoxy polymers that can be utilized.
Typically, advanced composite rims are comprised of what is known in the art as a laminate. A laminate comprises a plurality of relatively thin plies. Each ply comprises reinforcing fiber or fabric comprised of the reinforcing fiber oriented in a particular fashion. If the fibers of a ply are impregnated with semi-cured epoxy (or other polymeric resin), the ply is typically referred to as prepreg. One common form of prepreg often used to produce a wheel rim laminate has the fibers oriented unidirectionally.
To produce a laminate various plies are laid one on top of the other with the relative orientations of fibers of each ply potentially varying relative to the orientation of the fibers of other plies. Since reinforcing fibers exhibit most of their strength in the axial direction versus their transverse direction, the resultant properties of the laminate can be varied and tailored to a specific application.
Next, the laminate, or plurality of laminates, is placed within a mold that approximates the shape of the bicycle rim. The mold is then usually heated and pressure is applied to the laminate to compact the various plies together and minimize or eliminate any voids, or air pockets, existing within the polymeric matrix. One common method of applying pressure is to inflate a bladder that has been placed inside the typically tubular laminate in such a matter that it compresses the laminate up against the walls of the mold. The temperature, time and pressure utilized to cure the polymeric resin will very with the particular resin chosen but temperatures of 200° F. to 350° F. are most common for epoxies. Cure often requires one to four hours depending on the amount of time required to heat the mold up to a desired temperature. Pressures are typically in excess of 50 psi and more preferably 90 psi to 150 psi for most epoxy materials. The resulting cured laminate will comprise approximately 60 to 75% fibers by weight with the remainder comprising the matrix resin.
It is to be appreciated the foregoing is only one methodology utilized to produce a composite bicycle rim. For instance, some manufacturers substitute expandable foam to provide pressure. The foam upon its cure typically becomes a structural element of the rim; whereas, the bladder may be removed from the rim's interior. In other methodologies, expandable elastomers, such as silicone rubber, can be utilized to provide pressure. Furthermore, a rim need not be produced as a single piece. It can be produced as to clamshell halves that are subsequently bonded together, or it can be produced as arcuate sections that are subsequently joined to form a hoop.
After the rim hoop has been cured and formed, spoke holes are drilled into the inner apex thereof. The number of holes is dependent upon the desired number of spokes that will be used to build a wheel. The common numbers of spoke holes typically utilized in a bicycle wheel include 16, 18, 20, 24, 28 and 32. In conjunction with the spoke holes, nipple access holes are drilled in the outwardly facing side of the rim through which a wheel builder can access and tighten the threaded nipples into which the threaded ends of the spokes are received.
Once a wheel is built, the end of each nipple rests and is it tensioned against the edge of a spoke hole. Because the spoke hole is drilled, its edge comprises a plurality of discontinuous fiber ends encased in a polymeric matrix, such as epoxy. Further, the process of drilling a spoke hole causes the matrix material in the region of the spoke hole's edge to form micro cracks, which can, if overstressed, propagate and potentially cause the failure of the rim at this location. Essentially, drilled spoke holes accentuate one of the primary weaknesses of advanced composite materials: their inability to withstand concentrated and localized loading. By cutting the continuous fibers at the edges of the spoke holes during drilling each fiber behaves more like a cantilevered beam rather than a suspended beam. All things being equal, cantilevered beams can withstand much less force than a suspended beam. Further, the micro cracking in the matrix causes stress concentrations as the load or force is transferred between reinforcing fibers. These stress concentrations can cause the micro cracks to propagate and cause a condition known as delamination in the area surrounding the spoke hole significantly weakening the structure.
The problems associated with the drilled spoke holes are typically not a concern in relation to the nipple access holes that are located directly opposite the spoke holes through the top end or tire bed of the rim. This is because, unlike the spoke holes, the nipple access holes are relatively unstressed and are not subject to localized point loading.
To combat the spoke hole problem, composite rim manufacturers add additional plies of material in the region of the spoke holes to increase the rim's bearing strength. This, of course, increases the overall weight of the rim. However, increasing the number of plies or thickness of the rim in the spoke hole region does not prevent the creation of micro cracking during drilling. Accordingly, it is not unheard of for these types of rims with extra reinforcing to eventually fail as micro cracks grow and form strength-robbing delaminations. In short, additional reinforcement does mitigate the drilled spoke hole problem somewhat and accordingly extend the life of the rim, but it is not eliminate the problem.
Another problem associated with composite rims is braking. In road bikes, brake calipers are utilized that force friction inducing pads against the sidewalls of a rim. The friction between the pads and the rim facilitate the deceleration of the associated bicycle. Commonly utilized carbon fiber epoxy advanced composite materials tend to have coefficients of friction that are lower than those of metals, such as aluminum. Accordingly, for a given application of braking force the stopping power of an advanced composite rim is reduced relative to an aluminum rim.
The braking surface of a typical advanced composite rim is formed during fabrication as the sides of the rim are pressed against the surface of the fabrication tool. Accordingly, the surface texture of the resultant rim typically matches the surface texture of the tool. Since most tools are comprised of metal, such as aluminum or steel, and are produced through the processes of machining, the braking surfaces of the rims tend to have a machined finish which comprises a multitude of very fine undulating peaks and valleys. Such a surface effectively reduces the contact area of the braking surface with a brake pad during use as initial contact between a brake pad and the braking surface occurs only at the peaks. As can be appreciated, the reduced effective braking surface area acts to reduce braking efficiency even further.
To ensure a smoother braking surface, some composite rim manufacturers will polish the surfaces of the tool that correspond to a rim's braking surfaces. Polishing is typically performed using abrasive grit paper and/or abrasive polishes that are applied by hand or with the assistance of handheld power tools. While polishing does on a localized scale remove the fine undulations, it can introduce macro undulations over the braking surface. While the magnitude of the undulations may only be thousands of an inch, it can effectively reduce the consistent application braking force and consequently reduce braking efficiency.